Pump arrangement

ABSTRACT

A pump arrangement for pumping a liquid, in particular a liquid contaminated with dirt particles, the pump arrangement having at least one working chamber ( 1, 2, 11, 12 ) and at least one delivery piston ( 3, 4, 13 ), and at least one inlet valve ( 7 ) and at least one outlet valve ( 8 ), and the delivery piston ( 3, 4, 13 ) being movable in order to suck liquid into the working chamber ( 1, 2, 11, 12 ) through the inlet valve ( 7 ) and to eject liquid out of the working chamber ( 1, 2, 11, 12 ) through the outlet valve ( 8 ), and the pump arrangement having a wear-monitoring device ( 40 ) for monitoring the wear on the inlet valve ( 7 ) and/or outlet valve ( 8 ) with at least one pressure sensor ( 41 ) and one pressure evaluation unit ( 42 ) for processing pressure measurement values output from the pressure sensor ( 41 ), wherein the pump arrangement, for the purpose of moving the delivery piston ( 3, 4, 13 ), has a drive fluid system ( 20 ) that can be operated with a drive fluid, and the pressure sensor ( 41 ) of the wear-monitoring device ( 40 ) is arranged to detect the pressure of the drive fluid in the drive fluid system ( 20 ).

INCORPORATION BY REFERENCE

The following documents are incorporated herein by reference as if fully set forth: Austrian Patent Application No. A246/2016, filed May 17, 2016.

BACKGROUND

The present invention relates to a pump arrangement for pumping a liquid, in particular a liquid contaminated with dirt particles, the pump arrangement having at least one working chamber and at least one delivery piston, and at least one inlet valve and at least one outlet valve, and the delivery piston being movable in order to suck liquid into the working chamber through the inlet valve and to eject liquid out of the working chamber through the outlet valve, and the pump arrangement having a wear-monitoring device for monitoring the wear on the inlet valve and/or outlet valve with at least one pressure sensor and one pressure evaluation unit for processing pressure measurement values output from the pressure sensor. Furthermore, the invention also relates to a method for monitoring the wear on at least one inlet valve and/or at least one outlet valve of a pump arrangement.

Pump arrangements for pumping liquids, in particular liquids contaminated with dirt particles, are used, for example, in order to pump waste water from sewers, storage vessels, etc., or to flush waste water channels, wellbores or the like. Dirt particles are often present here in the form of solid grains (e.g. sand or stones) or in the form of fibers. In particular, dirt particles from solids may have an abrasive action and cause wearing of the components of the pump arrangement, e.g. the inlet valves and/or outlet valves. If dirt particles are caught in the inlet and/or outlet valve upon closure of the inlet and/or outlet valve, it is possible for locally high flow velocities to develop. Further dirt particles flowing through the inlet and/or outlet valve may in this case lead to wear, and consequently impaired functioning, of the inlet valve and/or outlet valve. The wear on the inlet valves and/or outlet valves ultimately reduces the pumping capacity of the pump arrangement, for which reason the inlet valves and/or outlet valves have to be overhauled or even completely replaced if they become worn.

WO 2007/072385 A2 discloses a pump arrangement of the aforementioned type for flushing wellbores, with a delivery piston driven by a rotating crankshaft. The wear-monitoring device proposed according to said specification permits monitoring of the wear on the inlet valve and/or outlet valve by pressure and acceleration sensors arranged in the working chamber. A further pressure sensor is arranged in a high-pressure section downstream from the working chamber. The pressure and acceleration sensors are in direct contact with the liquid to be delivered. When pumping liquid, in particular liquid contaminated with dirt particles, the pressure and acceleration sensors are thus directly exposed to the abrasive action of the medium.

SUMMARY

The object of the invention is to make available an advantageous pump arrangement of the aforementioned type which is also suitable for pumping liquids that are heavily contaminated with dirt particles.

This is achieved with a pump arrangement having one or more features of the invention.

In the pump arrangement according to the invention, provision is made that the pump arrangement, for the purpose of moving the delivery piston, has a drive fluid system that can be operated with a drive fluid, and the pressure sensor of the wear-monitoring device is arranged to detect the pressure of the drive fluid in the drive fluid system.

It was found that the pressure measurement values of the drive fluid that are output by the pressure sensor during the pumping process in the case of a damaged inlet valve and/or outlet valve differ from pressure measurement values output by the pressure sensor in the case of an intact inlet valve and/or outlet valve. With the pump arrangement according to the invention, it is thereby possible to detect wear on the inlet valve and/or the outlet valve by indirect monitoring with the at least one pressure sensor arranged in the drive fluid circuit.

By detecting the pressure of the drive fluid in the drive fluid system, wear on the at least one inlet valve and/or the at least one outlet valve can thus be detected without the pressure sensor being in direct contact with the liquid, in particular liquid contaminated with dirt particles.

The drive fluid is expediently a liquid medium, in particular hydraulic oil. In principle, it would also be conceivable and possible to use a gaseous drive fluid, e.g. ambient air.

The drive fluid system could be designed as an open drive fluid system. An open drive fluid system could prove expedient, for example, when using ambient air as drive fluid, in which case ambient air is then preferably taken from the atmosphere and compressed and, after use in the drive fluid system, returned to the atmosphere.

Particularly preferably, the drive fluid system is a closed drive fluid system, in which the drive fluid is conveyed within a closed circuit. In such cases, the drive fluid system could thus also be designated as a drive fluid circuit.

Preferably, a drive piston is mounted movably in a drive cylinder of the drive fluid system and is mechanically coupled to the at least one delivery piston. The drive piston and the at least one delivery piston could be mechanically coupled to each other, for example, by a common piston rod. An articulated mechanical coupling of the drive piston and of the at least one delivery piston is also conceivable and possible. A direct interconnection of drive piston and delivery piston is also possible.

The delivery piston could also be designated generally as a displacement body. By the movement of the delivery piston, the volume of the working chamber is increased in order to suck liquid into the working chamber and reduced in order to eject liquid from the working chamber. The delivery piston is expediently mounted in a delivery cylinder so as to be movable therein, preferably linearly movable. The working chamber is then located in the delivery cylinder.

In preferred embodiments according to the invention, provision is made that the pump arrangement has two delivery pistons which are connected, preferably rigidly, to the or a drive piston mounted movably in a drive cylinder of the drive fluid system. Particularly preferable is that provision is made that the drive piston is arranged between the two delivery pistons. A rigid connection of the drive piston to the two delivery pistons permits a synchronous reciprocating motion of the delivery pistons and of the drive piston.

In one group of embodiments of the invention, provision can be made that the delivery piston is designed as a plunger. Plungers are distinguished by a simple design. The plunger uses its volume to reduce the volume of the working chamber and thereby displace the liquid that is to be delivered out of the delivery cylinder. If the delivery piston is designed as a plunger, a seal can be provided that is arranged in a fixed position on the delivery cylinder, in which case, during the pumping process, a movement of the delivery piston relative to the seal takes place.

In other embodiments according to the invention, provision is made that the delivery piston has at least one peripheral seal. In these embodiments, the seal is arranged, preferably in a fixed position, on the delivery piston, in which case a movement of the seal relative to the delivery cylinder takes place.

In the embodiments in which the delivery piston comprises a peripheral seal, provision is expediently made that the pump arrangement has two working chambers, the delivery piston separating a first of the working chambers and a second of the working chambers from each other, and each of the working chambers being assigned at least one inlet valve and at least one outlet valve. The delivery piston therefore expediently has a dual action, i.e. ensuring that one of the two working chambers is increased in size in order to suck liquid through the inlet valve, while the second working chamber is at the same time reduced in size in order to eject liquid through the outlet valve.

The pressure evaluation unit of the wear-monitoring device expediently has a memory for storing pressure measurement values, and a microprocessor for processing the pressure measurement values. The pressure evaluation unit could also be integrated in a control unit of the pump arrangement.

Provision is preferably made that the wear-monitoring device comprises a signaling unit for informing a user of the presence of wear on the at least one inlet valve and/or the at least one outlet valve, the wear having been determined by the pressure evaluation unit. The signaling unit could, for example, output visual and/or acoustic information to the user. In addition to outputting a warning concerning the presence of wear on the inlet valve and/or outlet valve, it is also conceivable that operating data, in particular visual operating data, concerning the state of the inlet valve and/or outlet valve are continuously output to the user.

In preferred embodiments, the pump arrangement according to the invention has at least one proximity switch for detecting an end position, preferably the end positions, of the delivery piston and/or of the or a drive piston mounted movably in a drive cylinder of the drive fluid system. By providing a proximity switch, the position of the delivery piston and/or of the drive piston, during the monitoring of wear on the inlet valve and/or the outlet valve, can be taken into account by the pressure evaluation unit when processing the pressure measurement values.

Particularly preferably, provision is made that the pump arrangement has exactly one pressure sensor for measuring the pressure of the drive fluid, the pressure sensor being arranged between a preferably volume-regulated drive fluid pump and a switching valve for controlling the direction of movement of the at least one delivery piston. The drive fluid pump and the switching valve are preferably component parts of the drive fluid system. In this way, a pressure sensor can be used for measuring the pressure of the drive fluid in a first direction of movement and a second direction of movement of the drive piston and/or delivery piston.

The present invention also relates to a method for monitoring the wear on at least one inlet valve and/or at least one outlet valve in a pump arrangement according to the invention.

Provision is preferably made here that the pressure evaluation unit forms at least one characteristic value in each case from pressure measurement values processed during a stroke operation of the delivery piston, and the characteristic values of a sequence of stroke operations are evaluated.

Particularly preferably, the pressure evaluation unit forms a first characteristic value from processed pressure measurement values during a stroke of the drive piston in a first direction of movement and forms a second characteristic value from processed pressure measurement values of a succeeding stroke of the drive piston in a second direction of movement counter to the first direction of movement, and the difference of the characteristic values is compared with a previously defined threshold value.

In a preferred method, provision is made that the first characteristic value is a gradient value of the pressure increase during the stroke of the drive piston in the first direction of movement, and the second characteristic value is a gradient value of the pressure increase during the stroke of the drive piston in the second direction of movement.

In other preferred methods, provision could be made that, from processed pressure measurement values during a stroke of the drive piston, the pressure evaluation unit forms a characteristic value which is compared with a reference value. The reference value is expediently determined in the case of intact inlet valves and/or outlet valves. The reference value could be determined, for example, the first time the pump arrangement is started up and could be stored in the pressure evaluation unit. The reference value could also be assigned a tolerance range. Provision could then be made that the presence of wear is detected only when the determined characteristic value lies outside the tolerance range surrounding the reference value.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and details of preferred embodiments of the invention are explained with reference to the drawings, in which:

FIG. 1 shows a schematic view of a first illustrative embodiment of a pump arrangement according to the invention;

FIG. 2 shows a schematic view of a second illustrative embodiment according to the invention, and

FIG. 3 shows a view of an example of a pressure profile in the drive fluid system according to FIG. 1 when wear occurs on one of the inlet valves.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Two illustrative embodiments of pump arrangements for pumping liquid, in particular liquid contaminated with dirt particles, are shown in FIGS. 1 and 2. A pump arrangement of this kind could be arranged, for example, on a sewer-cleaning vehicle. In the following description, therefore, the liquid, in particular contaminated with dirt particles, is designated as waste water, even though pump arrangements according to the invention, in particular also according to the illustrative embodiments shown here, can of course also be used for other liquids. Waste water often has abrasive dirt particles in the form of granular and/or fibrous substances. In other illustrative embodiments, the pump arrangement according to the invention could also be used to pump other mixtures of liquid and solid substances, e.g. liquid concrete.

In the first illustrative embodiment according to FIG. 1, the pump arrangement for pumping waste water has two delivery cylinders 5, 6, each with a respective working chamber 1, 2. A delivery piston 3 is arranged in a linearly movable manner in the delivery cylinder 5, and a delivery piston 4 is arranged in a linearly movable manner in the delivery cylinder 6. In this illustrative embodiment, the delivery pistons 3, 4 are designed as plungers. The working chamber 1, 2 is sealed off from the delivery cylinder 5, 6 and from the respective delivery piston 3, 4 by seals 16 arranged in a fixed state on the respective delivery cylinder 5, 6. The volume of the working chambers 1, 2 can be modified by a movement of the respective delivery piston 3, 4 relative to the respective delivery cylinder 5, 6.

Each working chamber 1, 2 is assigned a respective inlet valve 7 and outlet valve 8. The inlet valves 7 and outlet valves 8 are designed as spring-biased non-return valves and are known per se. If the at least one inlet valve 7 and/or the at least one outlet valve 8 is intact, one of the directions of passage of liquid is blocked in a closure position. In the opposite direction of passage, liquid can flow through the opened inlet valve 7 and/or outlet valve 8. The schematically illustrated valve plates of the inlet valves 7 and outlet valves 8 thus bear with spring pre-tensioning on a corresponding valve seat (cf. FIG. 1) in the closure position.

The inlet valves 7 are connected in a fluid-conducting manner to a suction port 9 of the pump arrangement. A suction line (not shown in any detail) for taking up waste water, or liquid in general, in particular liquid contaminated with dirt particles, can be attached to the suction port 9. The outlet valves 8 are connected in a fluid-conducting manner to an outlet port 10 of the pump arrangement. A line (likewise not shown in any detail) could lead away from the outlet port 10 to a storage container for waste water, for example a storage container arranged on a sewer-cleaning vehicle.

The pump arrangement has a drive fluid system 20 that can be operated with a drive fluid. Hydraulic oil, for example, can be used as drive fluid. In the illustrative embodiments shown here, the drive fluid system 20 is designed as a closed system, i.e. the drive fluid is conveyed in a circuit in the drive fluid system 20.

The drive fluid system 20 has a volume-regulated drive fluid pump 27 which can be driven, for example, by an electric motor 28. Instead of an electric motor, it would also be possible, for example, to use an internal combustion engine, a hydraulic motor or the like. The volume-regulated drive fluid pump 27, in the illustrative embodiment shown here, supplies a constant volumetric flow for moving a drive piston 21, which volumetric flow is pre-selected for example by the user via a control unit 30 of the pump arrangement. The drive piston 21 is mounted in a linearly movable manner in a drive cylinder 22 of the drive fluid system 20. The drive piston 21, having a peripheral seal, separates a first drive chamber 23 and a second drive chamber 24 from each other.

A switching valve 26 conveys the pressurized drive fluid alternately to the first drive chamber 23 or the second drive chamber 24, while drive fluid from the other of the two drive chambers 23, 24 is expelled and is collected in an oil pan 29 of the drive fluid system 20. The drive fluid collected in the oil pan 29 can subsequently be sucked in again by the drive fluid pump 27, with which the circuit of the drive fluid system 20 is closed. The switching valve 26 of the drive fluid system 20 can be activated by the control unit 30 of the pump arrangement.

The pump arrangement according to FIG. 1 moreover has two proximity switches 43 for detecting the end positions of the drive piston 21. The signal of the proximity switches 43 is evaluated by the control unit 30, and, when an end position is reached, the switching valve 26 is switched in order to reverse the direction of movement of the drive piston 21. In the illustrative embodiment according to FIG. 1, the proximity switches 43 are arranged on the delivery cylinders 5, 6. The end positions of the drive piston 21 are thus output to the control unit 30 indirectly via a signal from the corresponding proximity switch 43, which signal is caused by the presence of one of the delivery pistons 3, 4 at the respective end position.

The delivery pistons 3, 4 are rigidly connected to the drive piston 21. In this illustrative embodiment, the drive piston 21 is arranged between the delivery pistons 3, 4. With this arrangement, it is possible to permit a continuous delivery of waste water from the side with the suction port 9 to the side with the outlet port 10.

A movement of the delivery piston 3, caused by the movement of the drive piston 21, in a first direction of movement 31 leads to an increase in the volume of the working chamber 1. Waste water is sucked into the working chamber 1 via the suction port 9 and through the inlet valve 7 assigned to the working chamber 1. The valve plate of the inlet valve 7 is lifted from the valve seat by the underpressure arising in the working chamber 1, and liquid flows out from the suction port 9 into the working chamber 1. During a movement of the delivery piston 3 in the first direction of movement 31, the outlet valve 8 assigned to the first working chamber 1 is in the closure position and, if the outlet valve 8 is intact, prevents a return flow of waste water through the outlet port 10 into the working chamber 1.

At the same time as the movement of the delivery piston 3, a synchronous movement of the delivery piston 4 in the first direction of movement 31 also takes place on account of the rigid connection to the drive piston 21, and the volume of the working chamber 2 is reduced. Here, waste water is expelled from the working chamber 2 through the outlet valve 8, assigned to the working chamber 2, in the direction toward the outlet port 10. During a movement of the delivery piston 3 in the direction of movement 31, the inlet valve 7 assigned to the working chamber 2 is located in the closure position and, if the inlet valve 7 is intact, prevents a return flow of waste water in the direction toward the suction port 9. When an end position is reached, which is on the right in the illustrative embodiment shown in FIG. 1 and which is defined by the proximity switch 43 arranged on the delivery cylinder 6, the direction of movement of the drive piston 21 and of the delivery pistons 3, 4 is reversed by switching of the switching valve 26. During a movement of the delivery pistons 3, 4 in the second direction of movement 32 counter to the first direction of movement 31, waste water is sucked through the suction port 9 into the working chamber 2 and waste water is expelled from the working chamber 1 in the direction toward the outlet port 10.

The first drive chamber 23 and the second drive chamber 24 are each sealed off, by a seal 16, from the working chambers 1, 2, which are filled with waste water during normal operation.

The pump arrangement has a wear-monitoring device 40 for monitoring the wear on the inlet valve 7 and/or on the outlet valve 8. For this purpose, a pressure sensor 41 of the wear-monitoring device 40 is arranged to detect the pressure of the drive fluid in the drive fluid system 20. The pressure measurement values output from the pressure sensor 41 are processed by a pressure evaluation unit 42. The pressure sensor 41 is arranged between the drive fluid pump 27 and the switching valve 26. In this way, a single pressure sensor 41 can be used to detect the pressure of the drive fluid during the movement of the drive piston 21 in the first direction of movement 31 and to detect the pressure of the drive fluid in the second direction of movement 32. By arrangement of the pressure sensor 41 between the drive fluid pump 27 and the switching valve 26, it is possible to do without an additional pressure sensor 41. In other embodiments, it would also be conceivable and possible to measure the pressure at another place in the drive fluid system 20 as well, e.g. on the respective connection line between the first drive chamber 23 and the switching valve 26 and between the second drive chamber 24 and the switching valve 26.

The wear-monitoring device 40 has a signaling unit 44 for acoustically and/or visually informing a user of the presence of wear on the at least one inlet valve 7 and/or on the at least one outlet valve 8, as determined by the pressure evaluation unit 42.

If wear occurs on the at least one inlet valve 7 and/or the at least one outlet valve 8, the reliable sealing of the valve plate relative to the valve seat in the closure position is no longer ensured. In this case, a return flow of liquid through the inlet valve 7 or the outlet valve 8 may occur, as a result of which the performance of the pump arrangement, or the output rate per unit of time, is reduced. The wear on the at least one inlet valve 7 and/or the at least one outlet valve 8 can be monitored by the pressure evaluation unit 42 since, in the event of a return flow of liquid, e.g. during the decrease in volume of one of the working chambers 1, 2, from the affected working chamber 1, 2 as a result of a worn inlet valve 7, the pressure profile in the drive fluid is also influenced.

In the illustrative embodiment, provision is made that, in addition to pressure measurement values output by the pressure sensor 41, the signals of the proximity switches 43, which detect the end position of the delivery pistons 3, 4 or of the drive piston 21, are transmitted to the pressure evaluation unit 42 from the control unit 30. In this way, the measured pressure measurement values can be allocated to strokes in the first direction of movement 31 and the second direction of movement 32. Alternatively or in addition, it is also possible that the current switching state of the switching valve 26 is transmitted from the control unit 30 to the pressure evaluation unit 42 in order to allow the pressure measurement values to be allocated to the current direction of movement of the drive piston 21.

FIG. 3 is a diagram showing an example of a pressure profile of the drive fluid, which is detected with the pressure sensor 41. The pressure at a defined time is plotted on the ordinate, and the associated time is plotted on the abscissa of the diagram. The pressure profile 50 is indicated in a simplified form in order to illustrate the principle by which the pressure measurement values are processed by the pressure evaluation unit 42.

The pressure profile 50 has a distinct valley 57 at the start of each stroke of the working piston 21 in the first and second directions of movement 31, 32. The plotted time interval 53 marks the pressure profile plotted during one stroke of the working piston 21 in the first direction of movement 31, while the time interval 54 marks the pressure profile plotted during one stroke of the working piston 21 in the second direction of movement 32.

Proceeding from the pressure measurement values measured during a sequence of strokes, a characteristic value 51, 52 is formed, and the difference of the characteristic values 51, 52 of successive stroke operations is compared with a previously defined threshold value. In the illustrative embodiment, average gradient lines 55, 56 of the pressure profile 50 are determined during a respective time interval 53, 54. The average gradient line 55, 56 is determined by the pressure evaluation unit 42, for example by a regression calculation. Extremes at the start or at the end of a respective stroke can be left out of consideration here. A first characteristic value 51 is formed from the gradient value of the gradient line 55. For the time interval 54, the gradient value of the gradient line 56 is determined as second characteristic value 52.

It will be clear from the pressure profile 50 shown in FIG. 3 that the characteristic value 51, i.e. the gradient of the gradient line 55, in the time interval 53 is smaller than the characteristic value 52, i.e. the gradient of the gradient line 56 in the time interval 54. The pressure evaluation unit 42 determines the difference of the characteristic values 51, 52 and compares this difference to the previously defined threshold value. If the difference exceeds the previously defined threshold value, the user receives a warning message via the signaling unit 44. In other words, in this method, a characteristic value 51 formed from the processed pressure measurement values during a stroke in the first direction of movement 31 is compared with a characteristic value 52 formed from the processed pressure measurement values during a stroke in the second direction of movement 32. From a relative deviation of the characteristic values 51, 52 from each other, it is possible to infer the presence of wear on the at least one inlet valve 7 and/or outlet valve 8 and, for example, to provide the user with corresponding information via the signaling unit 44.

In the illustrative embodiment, the characteristic values 51, 52 are determined from average gradient lines 55, 56 of the pressure profile 50. It is conceivable and possible also to use other characteristic values for monitoring the wear on the inlet valve 7 and/or the outlet valve 8. For example, a maximum pressure determined during one stroke could be compared with a maximum pressure of a succeeding stroke.

Alternatively or in addition, provision could also be made that, from processed pressure measurement values during a stroke of the drive piston 21, the pressure evaluation unit 42 forms a characteristic value which is compared with a reference value. The reference value could be determined, for example, the first time the pump arrangement is started up and could be stored in the pressure evaluation unit 42. As has already been explained in the introduction, the reference value could also be assigned a tolerance range. In this context, provision could be made that the presence of wear is detected, or the abovementioned information on the presence of wear is sent to the user, e.g. via the signaling unit 44, only when the determined characteristic value lies outside the tolerance range surrounding the reference value.

FIG. 2 shows a second embodiment of a pump arrangement according to the invention. The structure of the drive fluid system 20 of the pump arrangement corresponds to that of the first illustrative embodiment, and therefore, in the explanations concerning the second illustrative embodiment, it is essentially only the differences from the first illustrative embodiment that are discussed. Otherwise, the explanations concerning the first illustrative embodiment also apply to the second illustrative embodiment. As regards the function of the inlet valves 7 and outlet valves 8, reference is also made to the explanations concerning the first illustrative embodiment.

The pump arrangement according to the second illustrative embodiment comprises a delivery piston 13, which has a peripheral seal 15 arranged in a fixed position on the delivery piston 13. The delivery piston 13 is mounted in a linearly movable manner in a delivery cylinder 14. The delivery piston 13 is rigidly connected to the drive piston 21 by a piston rod 18. The pump arrangement has two working chambers 11, 12, and the delivery piston 13 separates the first of the working chambers 11 and the second of the working chambers 12 from each other. Each of the working chambers 11, 12 is assigned an inlet valve 7 and an outlet valve 8.

During a movement of the delivery piston 13 in the direction of movement 31, the working chamber 11 decreases in size, while at the same time the volume of the working chamber 12 increases. The seal 17 separates the second working chamber 12 and the second drive chamber 24 of the drive cylinder 22 from each other. The movement of the drive piston 21 takes place analogously to the first illustrative embodiment, wherein the end position of the drive piston 21 is effected directly via the proximity switches 43 arranged on the drive cylinder 22.

In the illustrative embodiment according to FIG. 2, the first drive chamber 23 has a circular cylindrical shape. The second drive chamber 24 has an annular cylindrical shape, on account of the piston rod 18 inwardly delimiting the drive chamber 24. Therefore, during the operation with a volume-regulated drive fluid pump 27 delivering a constant volumetric flow for moving the drive piston 21, the stroke movement in the second illustrative embodiment is slower in the first direction of movement 31 than in the second direction of movement 32. That is to say, for a stroke movement in the first direction of movement 31, a longer time interval is needed than for a stroke movement in the second direction of movement 32. This can be taken into account by the pressure evaluation unit 42 when processing the pressure measurement values output by the pressure sensor 41. For example, when determining characteristic values 51, 52, a correction value that reflects the different configuration of the drive chambers 23, 24 could be used by the pressure evaluation unit 42 in order to take account of the different time intervals 53, 54 when monitoring for wear. Otherwise, the monitoring for wear on the at least one inlet valve 7 and/or outlet valve 8 by the wear-monitoring device 40 can take place analogously to the first illustrative embodiment.

KEY TO THE REFERENCE NUMBERS

1 working chamber

2 working chamber

3 delivery piston

4 delivery piston

5 delivery cylinder

6 delivery cylinder

7 inlet valve

8 outlet valve

9 suction port

10 outlet port

11 working chamber

12 working chamber

13 delivery piston

14 delivery cylinder

15 seal

16 seal

17 seal

18 piston rod

20 drive fluid system

21 drive piston

22 drive cylinder

23 first drive chamber

24 second drive chamber

25 seal

26 switching valve

27 drive fluid pump

28 motor

29 oil pan

30 control unit

31 first direction of movement

32 second direction of movement

40 wear-monitoring device

41 pressure sensor

42 pressure evaluation unit

43 proximity switch

44 signaling unit

50 pressure profile

51 characteristic value

52 characteristic value

53 time interval

54 time interval

55 gradient line

56 gradient line

57 valley 

1. A pump arrangement for pumping a liquid, the pump arrangement comprising at least one working chamber, at least one delivery piston, at least one inlet valve, and at least one outlet valve, the delivery piston being movable in order to suck liquid into the working chamber through the inlet valve and to eject liquid out of the working chamber through the outlet valve, a drive fluid system that is operable with a drive fluid that moves the delivery piston, a wear-monitoring device for monitoring wear on at least one of the inlet valve or outlet valve including at least one pressure sensor and one pressure evaluation unit for processing pressure measurement values output from the pressure sensor, and the pressure sensor of the wear-monitoring device is arranged to detect a pressure of the drive fluid in the drive fluid system.
 2. The pump arrangement as claimed in claim 1, further comprising a drive piston mounted movably in a drive cylinder of the drive fluid system, the drive piston being mechanically coupled to the at least one delivery piston.
 3. The pump arrangement as claimed in claim 2, wherein the pump arrangement has two delivery pistons which are connected to the drive piston that is mounted movably in the drive cylinder of the drive fluid system.
 4. The pump arrangement as claimed in claim 3, wherein the two delivery pistons are connected rigidly to the drive piston.
 5. The pump arrangement as claimed in claim 1, wherein the delivery piston is a plunger.
 6. The pump arrangement as claimed in claim 1, wherein the delivery piston has at least one peripheral seal.
 7. The pump arrangement as claimed in claim 6, wherein the pump arrangement has first and second working chambers, the at least one inlet valve comprises first and second inlet valves, the at least one outlet valve comprises first and second outlet valves, the delivery piston separates the first working chambers and the second working chambers from each other, and each of the working chambers is assigned a respective one of the first and second inlet valves and a respective one of the first and second outlet valves.
 8. The pump arrangement as claimed in claim 1, wherein the wear-monitoring device comprises a signaling unit for informing a user of a presence of wear on at least one of the at least one inlet valve or the at least one outlet valve, the wear having been determined by the pressure evaluation unit.
 9. The pump arrangement as claimed in claim 1, wherein the pump arrangement has at least one proximity switch for detecting an end position of the delivery piston.
 10. The pump arrangement as claimed in claim 2, wherein the pump arrangement has at least one proximity switch for detecting an end position of the drive piston mounted movably in the drive cylinder of the drive fluid system.
 11. The pump arrangement as claimed in claim 1, wherein the pump arrangement has exactly one pressure sensor for measuring the pressure of the drive fluid, the pressure sensor being arranged between a drive fluid pump and a switching valve for controlling a direction of movement of the at least one delivery piston.
 12. The pump arrangement as claimed in claim 11, wherein the drive fluid pump is volume-regulated.
 13. The pump arrangement as claimed in claim 1, wherein the pump arrangement is adapted to pump liquid contaminated with dirt particles.
 14. A method for monitoring wear on at least one of the at least one inlet valve or at least one outlet valve in the pump arrangement as claimed in claim 1, comprising forming with the pressure evaluation unit at least one characteristic value in each case from pressure measurement values processed during a stroke operation of the delivery piston, and evaluating the characteristic values of a sequence of stroke operations. 